44096_fm_ptg01_i-xii.indd 1Licensed to: CengageBrain User44096_fm_ptg01_i-xii.indd 1Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. This is an electronic version of the print textbook. Due to electronic rights restrictions,some third party content may be suppressed. Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. The publisher reserves the right to remove content from this title at any time if subsequent rights restrictions require it. Forvaluable information on pricing, previous editions, changes to current editions, and alternate formats, please visit www.cengage.com/highered to search by ISBN#, author, title, or keyword for materials in your areas of interest.Licensed to: CengageBrain UserTechnicians Guide to Programmable Controllers, Sixth EditionTerry R. Borden, Richard A. CoxVice President, Editorial: Dave GarzaDirector of Learning Solutions: Sandy ClarkAcquisitions Editor: Stacy MasucciManaging Editor: Larry MainSenior Product Manager: John FisherVice President, Marketing: Jennifer BakerMarketing Director: Deborah YarnellSenior Marketing Manager: Erin BrennanMarketing Coordinator: Jillian BordenSenior Production Director: Wendy TroegerProduction Manager: Mark BernardContent Project Manager: David BarnesSenior Art Director: David ArsenaultTechnology Project Manager: Joe PlissCover Image: Dainis Derics/www.shutterstock .comPrinted in the United States of America 1234567141312 2013, 2007 Delmar, Cengage LearningALL RIGHTS RESERVED. 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Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain User1000011020030041050060070100110120130140150160170000010020030040050060070100110120130140150160171000011020030041050060070100110120130140150160170What is a Programmable Logic Controller (PLC)?CHAPTER1ObjectivesAfter completing this chapter, you should have the knowledge to:Describe several advantages of a programmable logic controller (PLC) over hardwired relay systems.Identify the four major components of a typical programmable logic controller and describe the function of each.Defne the term discrete.Defne the term analog.Identify different types of programming devices.A programmable logic controller is a solid-state system designed to perform the logic functions previ-ously accomplished by components such as electromechanical relays, drum switches, mechanical tim-ers/counters, etc., for the control and operation of manufacturing process equipment and machinery.Even though the electromechanical relay (control relays, pneumatic timer relays, etc.) has served well for many generations, often, under adverse conditions, the ever-increasing sophistication and com-plexity of modern processing equipment requires faster acting, more reliable control functions than electromechanical relays and/or timing devices can offer. Relays have to be hardwired to perform a specifc function, and when the system requirements change, the relay wiring has to be changed or modifed. In extreme cases, such as in the auto industry, complete control panels had to be replaced since it was not economically feasible to rewire the old panels with each model changeover.It was, in fact, the requirements of the auto industry and other highly specialized, high-speed manu-facturing processes that created a demand for smaller, faster acting, and more reliable control de-vices.Theelectrical/electronicsindustryrespondedwithmodular-designed,solid-stateelectronic devices. These early devices, while offering solid-state reliability, lower power consumption, ex-pandability, and elimination of much of the hardwiring also brought with them a new language. The language consisted of AND gates, OR gates, NOT gates, OFF RETURN MEMORY, J-K fip fops, and so on.What happened to simple relay logic and ladder diagrams? That is the question the plant engineers and maintenance electricians/technicians asked the solid-state device manufacturers. Thereluctance of the end user to learn a new language and the advent of the microprocessor gave the industry what is now 44096_ch01_ptg01_001-009.indd 1 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain User2 Technicians Guide to Programmable Controllersknown as the programmable logic controller (PLC). The frst program mable logic controller was invented in 1969 by Richard (Dick) E. Morley, who was the founder of the Modicon Corporation.Internally there are still AND gates, OR gates, and so forth in the processor, but the design engineers have preprogrammed the PLC so that programs can be entered using RELAY LADDER LOGIC. While RELAY LADDER LOGIC may not have the mystique of other computer languages such as PASCAL, FORTRAN, and C++, it is a high-level, real-world, graphic language that is understood by most electricians and technicians. RELAY LADDER LOGIC programming is the most common programming language used today but other programming languages such as Sequential Function Chart, Structured Text, and Function Block languages can also be found.The National Electrical Manufacturing Association (NEMA) defnes a programmable controller as follows:A programmable controller is a digital electronic apparatus with a programmable memory for storing instructions to implement specifc functions, such as logic, sequencing, timing, count-ing, and arithmetic to control machines and processes.What does a PLC consist of, and how is it different from a computer system? The PLC consists of a programming device (computerorhandheldprogrammer), processor unit, power supply, and an input/output (I/O) interface such as the computer system illustrated in Figure 11. And while there are similarities, there are also some major differences.Figure 11 Comparison of a Computer System and a PLCCPU(CENTRAL PROCESSORUNIT)MEMORYMEMORYCPUHARD DRIVEPOWER SUPPLYPOWER SUPPLYMONITORPROGRAMMING DEVICEOUTPUTMODULEINPUTMODULEKEYBOARDFLOPPY DRIVEINPUTPRINTEROUTPUTMCOMPUTERPLC Cengage Learning 201344096_ch01_ptg01_001-009.indd 2 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain UserWhat is a Programmable Logic Controller (PLC)? 3Note:An interface occurs when two systems come together and interact, or communicate. In the case of the PLC, the communication or interaction is between the inputs (limit switches, push buttons, sen-sors, and the like), outputs (coils, solenoids, lights, and so forth), and the processor. This interface happens when any input or output voltage (AC or DC) or current signal is changed to or from a low- voltage DC signal that the processor uses internally for the decision-making process.PLCs are designed to be operated by plant engineers and maintenance personnel with limited knowl-edge of computers. Like the computer, which has an internal memory for its operation and storage of a program, the PLC also has a memory for storing the user program, or LOGIC, as well as a memory for controlling the operation of a process machine or driven equipment. But unlike the computer, the PLC is typically programmed in RELAY LADDER LOGIC, not one of the computer languages. It should bestated, however, that some PLCs will use other forms of PLC language, such as Structured Text, Sequential Function Chart, and Function Block to program the PLC. A brief description of Structured Text, Sequential Function Chart, and Function Block programming will be provided in later chapters.The PLC is also designed to operate in the industrial environment with wide ranges of ambient tem-perature, vibration, and humidity, and is not usually affected by the electrical noise that is inherent in most industrial locations.Note:Electrical noise is discussed in Chapter 2.Maybe one of the biggest, or at least most signifcant, differences between the PLC and a computer is that PLCs have been designed for installation and maintenance by plant electricians who are not requiredtobehighlyskilledelectronicstechnicians.TroubleshootingissimplifedinmostPLCs because they include fault indicators, blown-fuse indicators, input and output status indicators, and written fault information that can be displayed on the programmer.Although the PLC and the personal computer are different in many ways, the personal computer is often used for programming and monitoring the PLC. Using personal computers in conjunction with PLCs will be discussed in later chapters.A typical PLC can be divided into four components. These components consist of the processor unit, the power supply, the input/output section (interface), and the programming device.The processor unit houses the processor, which is the decision-maker, or brain of the system. The brain is a microprocessor-based system that replaces control relays, counters, timers, sequencers, and so forth, and is designed so that the user can enter the desired program in RELAY LADDER LOGIC. Theprocessorthenmakesallthedecisionsnecessarytocarryouttheuserprogram,basedonthe status of the inputs and outputs for control of a machine or process. It can also perform arithmetic functions, data manipulation, and communications between the local I/O section, remotely located I/O sections, and/or other networked PLC systems. Figure 12 shows Allen-Bradley SLC-5/05 and PLC-5/20C, and LOGIX 5550 processor units.Note:Some manufacturers refer to the processor as a CPU or central processing unit.The power supply is necessary to convert 120 or 240 volts AC voltages to the low voltage DC re-quired for the logic circuits of the processor, and for the internal power required for the I/O modules. 44096_ch01_ptg01_001-009.indd 3 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain UserThe power supply can be a separate unit as, shown in Figure 13, one of modular design that plugs into the processor rack, as shown in Figure 14, or, depending on the manufacturer, one that is an integral part of the processor.Note: Thepowersupplydoesnotsupplypowerfortheactualinputoroutputdevicesthem-selves; it only provides the power needed for the internal circuitry of the input and output mod-ules. DC power for the input and output devices, if required, must be provided from a separate source.The power supply can be broken down into four basic parts as shown in Figure 15. The frst block, orsection,ofthepowersupplyconsistsofastep-downtransformer.Thestep-down transformer 4 Technicians Guide to Programmable ControllersFigure 12 Allen-Bradley SLC-5/05 andPLC-5/20C, and LOGIX 5550 Processor UnitsCourtesy of Allen-Bradley Co., Inc.44096_ch01_ptg01_001-009.indd 4 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain User reducesthevoltageleveloftheincomingACpower.Manypowersuppliesuseastep-down transformer that is also a constant voltage transformer. A constant voltage transformer maintains a constant output voltage, even if the incoming power is fuctuating. The second portion of the power supply is the rectifer section, and contains the full wave bridge rectifer(s) to convert the AC sine wave from the secondary of the transformer to a pulsating DC voltage (shown by the wave form inFigure15).ThepulsatingDCvoltagemustbefurtherconditionedbeforeitcanbeusedby the processor and I/O modules. The third section of the power supply, the flter section, uses flter Figure 15 Block Diagram of a Typical Power SupplyPOWER SUPPLYINPUT120240 V ACREDUCEDVOLTAGE ACPULSATINGDC VOLTAGE DC VOLTAGESTEP-DOWNTRANS-FORMERSECTION1SECTION2SECTION3SECTION4FULLWAVERECTI-FIER(S)FILTERNET-WORKVOLT-AGEREGU-LATOR5 V DC OUTPUTWhat is a Programmable Logic Controller (PLC)? 5Figure 14 Modular (Plug-in) Power SupplyCourtesy of GE Fanuc AutomationCourtesy of Allen Bradley Co., Inc.Figure 13 Allen-Bradley SeparatePower Supply Cengage Learning 201344096_ch01_ptg01_001-009.indd 5 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain Userdevices and/or networks to flter and smooth the DC voltage coming from the rectifer section. The fnal section of the power supply consists of a voltage regulator. The regulators function is to main-tain a constant DC output voltage, even if the incoming AC voltage fuctuates or varies due to load changes or line disturbances.The size or amperage rating of the power supply is based on the size, number, and type of I/O modules that are to be used. Power supplies are normally available with output current ratings of 320 amps.Note:Consider future needs and the possibility of expansion when initially sizing the power supply. It is cheaper in the long run to install a larger power supply initially than to try to add additional capacity at a later date.The input/output section consists of input modules and output modules. The number of input and output modules necessary is dictated by the requirements of the equipment that is to be controlled by a PLC. Figure 16 shows an input/output section. Modules are plugged in or added as required.Input and output modules, referred to as the I/O (I for input and O for output), are where the real-world devices are connected. The real-world input (I) devices can be push buttons, limit switches, analog sensors, pressure switches, selector switches, etc., while the real-world output (O) devices can be hardwired motor starter coils, solenoid valves, indicator lights, po sitioning valves, and the like.Thetermrealworldisusedtodistinguishactualdevicesthatexistandmustbephysically wired from the internal functions of the PLC system that duplicate the function of relays, timers, counters, and so on, even though none physically exists. This may seem a bit strange and hard to 6 Technicians Guide to Programmable ControllersFigure 16 Inserting a 32-Point Input Module into a Modicon I/O Rack Courtesy of Modicon Inc.44096_ch01_ptg01_001-009.indd 6 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain Userunderstand at this point, but the distinction between what the processor can do internallywhich eliminates the need for all the previously used control relays, timers, counters, and so forthwill be graphically shown and readily understandable later in the text.Real-world input and output devices are of two types: discrete and analog. Discrete I/O devices areeither ON or OFF, open or closed, while analog devices have a range of possible values. Examples of discrete devices are limit switches, push buttons, motor starter coils, and indicator lamps. Examples of analog devices are pressure sensors, temperature probes, panel meters, variable speed drive signals, and modu-lating valves. When reference is made to an I/O device, the terms discrete input device, discrete output device, analog input device, and analog output device are commonly used to describe the type of device.A reference was made earlier in this chapter to the I/O section as an interface. Although not a com-monreference,itisanaccurateone.TheI/Osectioncontainsthecircuitrynecessarytoconvert input voltages of 120240 V AC or 024 V DC, etc., from discrete input devices to low-level DC voltages (typically 5 V) that the processor uses internally to represent the status or condition (ON or OFF). Similarly, the I/O section changes low-level DC signals from the processor to 120240 V AC or DC voltages required tooperate the discrete output devices. The I/O section also converts varying voltage or current signals from analog input devices into corresponding decimal values by wayofanAnalog-to-Digitalconverter(ADC).Thissameprocess,butreversedviaaDigital-to-Analog converter (DAC), is used by the I/O section to convert decimal values into corresponding voltage or current signals necessary to operate analog output devices. The feld signals from both digital and analog devices are normally isolated from the low-level logic circuitry of the processor by means of optical coupling. This is a brief overview of the I/O section and its function. How input andoutputdevicesarewiredtoI/Omodules,opticalcoupling,andmoreinformationaboutthe module circuitry itself is covered in Chapter 2.Theprogrammingdeviceisusedtoenterthedesiredprogramorsequenceofoperationintothe PLC memory. The program is entered using RELAY LADDER LOGIC, or one of the other PLC programming languages, and it is this program that determines the sequence of operation and ulti-mate control of the process equipment or driven machinery. The programming device can be one of two types: hand-held or personal computer. The personal computer, or PC, is the most common programming device used today. The dedicated hand-held programmer (Figure 17) was once very popular but has been largely replaced with portable personal computers.A personal computer (PC) is used to program most of the PLCs on the market today. The PLC programming software that is installed on the PC and a communications cable is sometimes all that is required to program the PLCs. At other times special hardware keys and/or communica-tioncardsarerequiredtobeinstalledonthePCforittoworksuccessfullyasaprogramming device. The PC provides the beneft ofa large viewing screen that allows more of the program tobeviewedatonetimeandmakestroubleshootingandmemoryaccessmucheasier.Italso providesprogramstorage,aswellasrunsallthevarioussoftwarepackageswehavecometo dependontoday,suchasspreadsheets,wordprocessing,andgraphics.Figure18showsa laptop personal computer that, with the appropriate software, is used to program and monitor a programmable logic controller.What is a Programmable Logic Controller (PLC)? 744096_ch01_ptg01_001-009.indd 7 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain UserFigure 17 Hand-Held Programmer Figure 18 Laptop Computer Connected to a AB MicroLogix 10008 Technicians Guide to Programmable ControllersCourtesy of Modicon Inc. Cengage Learning 201344096_ch01_ptg01_001-009.indd 8 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.Licensed to: CengageBrain UserChapter SummaryProgrammable logic controllers (PLCs) have made it possible to precisely control large process ma-chines and driven equipment with less physical wiring and lower installation costs than is required with standard electromechanical relays, pneumatic timers, drum switches, and so on. The program-mability allows for fast and easy changes in the RELAY LADDER LOGIC to meet the changing needs of the process or driven equipment without the need for expensive and time- consuming rewir-ing. Designed to be technician friendly, the modern PLC is easier to program and can be used by plant engineers and maintenance technicians who have little or no electronic background.Review Questions1.List the four main components of a programmable logic controller.2.Defne the term interface.3.Defne the term real world.4.Defne the term discrete.5.Explain the following initials or acronyms:DCADCACCPUDACPCPLCNEMAI/O 6.Defne the term analog.7.List the two types or styles of programming devices.8.RELAY LADDER LOGIC is a high-level graphic computer language.TF9.What is the major advantage of a PLC system over the traditional hardwired control system? 10. Draw a block diagram and label the main components of a typical DC power supply.What is a Programmable Logic Controller (PLC)? 944096_ch01_ptg01_001-009.indd 9 12/29/11 8:50 PMCopyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.